iStockphoto/Thinkstock(WASHINGTON) -- The Internal Revenue Service's explanations for why it targeted conservative groups for additional scrutiny are falling on deaf ears in Congress, where lawmakers like Senate Minority Leader Mitch McConnell are in no mood for what he called the agency's "blatant and thuggish abuse of power."

The Kentucky Republican also questioned Tuesday whether some of the blame should fall at President Obama's doorstep.

"What we don't know at this point is whether it jumped the fence from the IRS to the White House," McConnell said on the Senate floor. "But we do know this: We can't count on the administration to be forthcoming about the details of this scandal, because so far they've been anything but."

"So, this morning, I'm calling on the president to make available, completely and without restriction, everyone who can answer the questions we have as to what was going on at the IRS, who knew about it and how high it went," he continued.

Acting IRS commissioner Steven Miller learned of the practice in May 2012 and the agency has since fixed the problem, officials said Monday.

Still, lawmakers want to know why the IRS failed to own up to the problem in response to numerous congressional inquiries in 2012.

In several letters to members of Congress in 2012, including one from Miller just before the election in September, IRS officials made no mention of their knowing of the inappropriate targeting of tea party groups by IRS officials.

Around that same time in 2012, Senate Democrats were putting pressure on the IRS to beef up scrutiny of tax-exempt groups that "focused on federal election activities."

In a letter signed by seven Democrats, including Sens. Chuck Schumer, D-N.Y., and Al Franken, D-Minn., the senators requested that the IRS "immediately change the administrative framework for enforcement of the tax code as it applies to groups designated as 'social welfare' organizations."

"We urge the IRS to take these steps immediately to prevent abuse of the tax code by political groups focused on federal election activities," they wrote.

Around that time, the IRS admits to have targeted conservative groups with the names "Tea Party" or "Patriot" in their names for additional scrutiny, which in many cases delayed approval and potentially discouraged some groups from seeking tax exempt status.

When asked about the accusations Monday, President Obama said he first learned through media reports about the practice of targeting conservative group at the IRS Friday.

Questions now extend to whether IRS officials in California were involved in targeting conservative groups, despite the IRS' claims that the problem occurred as a result of the actions of a small group of "low level" employees in a Cincinnati office.

In at least one case, a conservative group received letters from IRS officials in a Redding, Calif., office.

Before his congressional testimony Friday, commissioner Miller acknowledged that his agency made mistakes in handling a flood of applications for tax-exempt status between 2010 and 2012.

But he insisted that the mistakes were not influenced by a political effort to stymie conservative or tea party groups.

"Mistakes were made, but they were in no way due to any political or partisan motivation," Miller said in an editorial on Monday night. "We are -- and will continue to be -- dedicated to reviewing all applications for tax-exempt status in an impartial manner."